The invention relates generally to a ferrofluid seal for a shaft, and more particularly to a seal comprising several sealing gaps that lie axially displaced from each other. Each gap is associated with a bore-hole for the radial supply of ferrofluid liquid.
An example of a prior art ferrofluid seal is set forth in U.S. Pat. No. 4,598,914. In this prior art device, a shaft is surrounded by an annular magnet, the internal diameter of which is slightly greater than the shaft diameter. A wishbone-like pattern is cut into the magnet surface facing the shaft. The sealing gap between the magnet and the shaft is sealed with ferrofluid liquid. This fluid is supplied to the sealing gap at a centrally located place within the wishbone pattern by means of a bore-hole. After being charged with ferrofluid, the bore-hole is closed off to the outside. The direction of rotation of the shaft is selected so that it cooperates with the wishbone pattern in generating radial pressure in the ferrofluid. This pressure is at a maximum in the central region of the annular magnet. In this manner, the resulting pressure rise in the central region enables the device to transmit bearing forces to the shaft. A fundamental precondition for this device to operate as desired is that the clearance spaces of the sealing gaps first be filled with ferrofluid liquid, and that the total volume of ferrofluid be accumulated at this location. The pressure retaining capacity of this ferrofluid seal is not very satisfactory.
A second prior art example of a ferrofluid seal for a shaft element is provided by U.S. Pat. No. 4,526,382. In the device of this patent, several sealing gaps displaced from each other in the axial direction circumferentially surround a shaft. The individual sealing gaps are separated from each other by free spaces, through which the ferrofluid liquid contained in the individual sealing gaps is activated independently from each other. The resulting sealing arrangement operates as a spaced apart, multiple-stage arrangement. This structural arrangement provides the ferrofluid seal with an improved pressure retaining capacity. However, it is extremely difficult to supply the ferrofluid liquid to the individual sealing gaps with sufficient dosing accuracy. This can only be done by using complicated auxiliary devices. When a loss of ferrofluid liquid occurs during normal operation, the entire ferrofluid seal has to be disassembled and repaired.
Hence, there remains a need for a ferrofluid seal that features good pressure retaining capacity and which greatly simplifies the procedure of introducing the ferrofluid liquid into the sealing gaps so that the device can more easily be employed and maintained.